The present invention relates to an electron beam focusing system that is used for a transmission-type electron microscope to focus the electron beam from the electron gun before the beam impinges (irradiates) upon a specimen.
Observations of diffraction images produced by focused electron beams using an electron microscope have been widely made. FIG. 1 shows an electron emission and focusing system which is used for a conventional electron microscope or the like to observe a diffraction image produced by a focused electron beam. This system includes an electron gun 1 that produces an electron beam 2, which is focused by a first stage focusing (condenser) lens 3. The focused beam then passes through a movable diaphragm (aperture plate) 4, and is focused by a second stage focusing lens 5. Thereafter, the beam is focused by a front magnetic field lens 6a in an objective lens 6, and then it impinges upon a specimen 7. The beam is diffracted by the specimen 7 and brought to focus by means of a rear magnetic field lens 6b in the objective lens and an imaging lens system (not shown) that is mounted at the rear of the lens 6b. Thus, the beam is projected onto a fluorescent screen 8, focusing a diffraction (pattern) image. To observe such a diffraction image, the following requirements must be met.
(1) In order to read information as much as possible from a diffraction disk 9 projected onto the fluorescent screen 8, the diameter d of the disk must be maximized in such a way that neighboring diffraction disks are not superimposed on each other.
(2) In order that the obtained diffraction electron beam come from a perfect region of a crystal, the diameter of the spot of the beam falling on the specimen must be kept at an initially set, small value.
The diameter d of a diffraction disk depends on the divergence angle 2.alpha. of the electron beam that irradiates the specimen 7. Accordingly, the divergence angle 2.alpha. is required to be adjusted to satisfy the aforementioned requirements. For simplicity, .alpha. that is half the divergence angle will be hereinafter referred to as divergence angle.
As one conceivable method of adjusting the divergence angle .alpha., the excitation magnitudes of focusing lenses are adjusted. However, the aforementioned prior art electron emission and focusing system is equipped with only two stages of focusing lenses. Therefore, if the excitation magnitudes of two stages of focusing lenses are varied, the total magnification of these two lenses will change, leading to a change in the diameter of the spot of the electron beam. As a result, requirement (2) above will not be fulfilled. For this reason, the common practice adopted to adjust the divergence angle is to exchange the movable diaphragm for another and alter the diameter of the aperture without changing the excitation magnitudes of the focusing lenses. This makes it impossible to adjust the divergence angle finely. Thus, it has not been possible to observe an electron beam diffraction image by magnifying a diffraction disk to an appropriate size. Although a focusing lens system equipped with three stages of focusing lenses actually exists, the focusing lenses whose excitation magnitudes are varied are only two. Consequently, this system has substantially the same problems as the two-stage focusing lens system. These are described in "Introduction to Analytical Electron Microscopy" Plenum Press, 1979, Chapters 14 and 15.